The present invention relates generally to the field of semiconductor devices and in particular the invention provides an improved method for the formation of crystalline films on glass substrates.
Considerable attention internationally is being directed towards developing technology for depositing polycrystalline silicon films on glass. This interest arises from the use of these films for forming electronic devices and circuits for active matrix liquid crystal displays and for use of these films in solar cells. Silicon film thickness could be in the 30 nm to 100 xcexcm range depending on application. For solar cell use, silicon film thickness in the 0.5-100 xcexcm range is of particular interest, with optimal designs likely to be in the 1-5 xcexcm range.
As part of these efforts, considerable attention is being directed to the development of supporting glass substrates with specific properties, particularly in relation to the thermal expansion coefficient of the glass relative to silicon and the strain point of the glass (temperature at which viscosity reaches a value of 1014.5 poise). For example, Corning Glassworks have developed a barium alumina silicate glass known as Corning 1737 of strain point above 660!C with a thermal expansion coefficient closely matched to silicon below this temperature for use in active matrix liquid crystal displays; the Max Planck Institute in Stuttgart has developed a glass of unspecified composition which has a strain point of 820!C and a thermal expansion coefficient even better matched to silicon below this temperature for use in solar cells.
A surprising conclusion from research of the present inventors is that this earlier work is heading in the wrong direction. Because amorphous silicon shrinks irreversibly when crystallised, use of a high-strain point glass as a substrate will result in a highly stressed film prone to cracking, even if it is perfectly expansion matched. The present invention arises from the realisation by the inventors that the normal soda lime glasses, developed over the centuries largely to produce durable glass which could be manufactured at low processing temperatures, and hence having low strain points, are ideal for this application. Throughout this specification, the term xe2x80x98amorphous siliconxe2x80x99 is used to describe silicon and silicon alloys containing a high proportion of amorphous silicon such that the material displays the shrinkage characteristic of amorphous silicon material upon its crystallisation. However, the material may include a proportion of crystalline silicon (eg Crystalitec) as well as alloying substances and impurities.
According to a first aspect, the present invention provides a method of forming a thin film of crystalline semiconductor material on a glass substrate, including the steps of:
depositing a film of the semiconductor material in amorphous form over the glass substrate;
processing the semiconductor material to form crystalline semiconductor material;
during or subsequently to the processing step, heating the substrate and semiconductor material to a temperature at or above the strain point temperature of the substrate;
subsequently to the heating step, cooling the substrate and semiconductor material to a temperature below the strain point of the substrate.
According to a second aspect, the present invention provides a method of forming a thin film of crystalline semiconductor material on a glass substrate, including the steps of:
a) heating the glass substrate to a temperature at which deposition of the crystalline semiconductor material may occur;
b) depositing a film of the crystalline semiconductor material over the glass substrate;
c) during or subsequently to the depositing step, heating the substrate and semiconductor material to a temperature at or above the strain point temperature of the substrate;
d) subsequently to the heating step, cooling the substrate and semiconductor material to a temperature below the strain point of the substrate.
According to a third aspect, the present invention provides a method of forming a thin film of crystalline semiconductor material on a glass substrate, including the steps of:
a) heating the substrate to a temperature at or above the strain point temperature of the substrate;
b) after the heating step and while the substrate is still at or above the strain point temperature depositing a film of the crystalline semiconductor material over the glass substrate;
c) subsequently to the deposition step, cooling the substrate and semiconductor material to a temperature below the strain point of the substrate.
According to a fourth aspect, the present invention provides a method of processing an amorphous semiconductor film on a glass substrate to crystallise the film, the method including the steps of:
a) processing the semiconductor material to form crystalline semiconductor material;
b) during or subsequently to the processing step, heating the substrate and semiconductor material to a temperature at or above the strain point temperature of the substrate;
c) subsequently to the heating step, cooling the substrate and semiconductor material to a temperature below the strain point of the substrate.
According to a fifth aspect, the present invention also provides a method of forming a thin film of crystalline semiconductor material supported by a glass substrate, including the steps of:
a) forming a low strain point temperature film over the substrate;
b) depositing a film of amorphous semiconductor material over the low strain point temperature film;
c) processing the semiconductor material film to form crystalline semiconductor material film.
According to a sixth aspect, the present invention also provides a device manufactured according to any one of the above methods.
According to a seventh aspect, the present invention further provides a semiconductor device including a film of crystalline semiconductor material formed on a glass substrate the substrate having a strain point temperature below the desired crystallisation temperature of the semiconductor material and a temperature co-efficient not less than that of the semiconductor material.
Preferably the substrate is a glass having a strain point temperature below the desired crystallisation temperature of the semiconductor material.
In this context desired crystallisation temperature is the temperature at which crystallisation occurs to achieve desired crystalline characteristics. In one method according to the invention, during or after the crystallisation step the substrate is heated to a temperature where it will deform under gravity against a planar form, within a predetermined processing period, to reverse buckling caused by differential stress between the substrate and the semiconductor film. This temperature will be somewhere between the strain point and the working point of the glass, the temperature used being dependent upon the speed with which the glass is required to flatten. Using this method the substrate can also be placed on a shape form to produce purpose shaped panels for applications such as vehicle sun roofs. In the case of soda lime glass the temperature used with good effect in one embodiment of the invention is 650xc2x0 C.
In another method according to the invention, during or after the crystallisation step the substrate is heated to a temperature at or above the strain point temperature but below the sagging temperature. The sagging temperature is the temperature where it will deform under gravity within a predetermined processing period. Using this alternative method films can be successfully processed with a maximum temperature of 620xc2x0 C. This method may be performed with the substrate clamped to a supporting form to reduce buckling.
In another method according to the invention the cooling step includes a step of rapidly cooling the surface of the substrate carrying the semiconductor film. Preferably also the cooling step includes a step of rapidly cooling the surface of the substrate opposite to the surface carrying the semiconductor film. In this context rapid cooling is intended to indicate cooling at a rate at which the glass is not substantially isothermal whereby the glass is also tempered by the cooling process. For 3 mm soda lime glass a rate slower than in the order of 0.5xc2x0 C./sec would be considered isothermal. However this value will vary according to the type of glass and its thickness. The rate will vary approximately as the square of thickness such that for 1 mm glass isothermal cooling will occur at rates slower than in the order of 5xc2x0 C./sec while for 10 mm thick glass isothermal cooling will occur at rates slower than in the order of 0.05xc2x0 C./sec. Throughout this specification, cooling which is referred to as greater than the rate at which isothermal cooling will occur will be taken to be at a rate at or above a rate for the particular glass thickness which corresponds to those rates given above for 1, 3 and 10 mm glass respectively, and which results in a significant temperature gradient across the thickness of the glass during the cooling process.
In the case of 3 mm soda lime glass a typical cooling rate in the region from 20xc2x0 C. above the strain point to 20xc2x0 C. below the strain point would be such that the transition took in the order of 10 seconds (ie 4xc2x0 C./sec) but rates in the range 0.5xc2x0 C. to 10xc2x0 C./sec can be applied to good effect. At temperatures further from the strain point cooling may take place at higher or lower rates depending upon other processing requirements.
In one embodiment of the invention, the surface of the substrate on which the semiconductor film is formed is modified to make it more fluid at low temperatures. The surface may be modified by the addition or removal of selected chemical species or by high energy irradiation. Alternatively a low strain point layer can be deposited onto the substrate surface prior to formation of the semiconductor film.
Preferably the semiconductor film is a film of doped or undoped silicon or silicon alloy material. In one application of the invention the silicon film is formed as a plurality of different doped layers of silicon forming one or more rectifying junctions arranged as a photovoltaic device, or solar cell.
The silicon film may be formed by methods such as plasma enhanced chemical vapour deposit or sputtering amongst others.
Silicon film thicknesses with a lower limit of 0.1 xcexcm can be employed in various embodiments covering a range of applications however preferably in photovoltaic applications films with a lower limit of 0.5 xcexcm and most desirably a lower limit of 1 xcexcm are employed.
A practical upper limit for film thickness is 100 xcexcm however for solar cell devices a preferred upper limit of film thickness is 151 xcexcm.
The method of invention can be performed with glasses having a strain point below the melting point of silicon, however it is more desirable that the substrate have a strain point below or at least not significantly above the desired crystallisation temperature of the silicon film (approx 600xc2x0 C.).
The best effect of the invention is achieved when the substrate has a temperature co-efficient not less than that of the crystalline semiconductor material, at least for temperatures below the strain point temperature of the substrate.
Preferably the substrate will be soda lime glass or a similar glass having a strain point temperature at or below 520xc2x0 C., an annealing point of approximately 550xc2x0 C. and a temperature co-efficient of 4-10 ppm/xc2x0 C.
Soda lime glasses with a composition of 70-75% by weight SiO2, 10-20% Na20, 3-15% CaO and less than 0.2% by weight Fe2O3 have been found to be effective in performing the invention while low iron glasses having less than 0.1% Fe2O3 have been found to be particularly advantageous when the approach is used to form solar cells. Preferably, glass with a thickness in the range of 2-4 mm will be used.
In one alternative embodiment, an intermediate layer is formed between the substrate and the semiconductor film, the intermediate layer having a low strain point and a thickness in the range of 0.1-10 xcexcm. As well as providing strain relief at lower temperatures the intermediate layer can be used to act as a chemical barrier layer and/or anti reflection layer. Intermediate layers such as nitride layers, that do not contribute strain relief at low temperatures can also be included without substantially interfering with the process described.
In the embodiments described above and in the following detailed description, the glass substrate may become the superstrate in a final product. For example, in the case of a solar cell, the cell may be illuminated through the glass layer.